The present invention relates to a gas generation system for providing a gas flow to be supplied to a reformer, with the gas flow comprising at least one carbon compound, such as hydrocarbon or alcohol, and water vapor. An evaporator for evaporating at least one of the components supplied to the reformer is provided. The present invention also relates to a corresponding method for providing a gas flow to a reformer.
Gas generation systems of this type are used, for example, to supply hydrogen as a fuel to fuel cell systems. Hydrocarbons or alcohols, such as methanol, together with water vapor are supplied to the reformer in gaseous form, where a catalytic conversion to a reformate is performed. The reformate contains essentially hydrogen, carbon dioxide, water (vapor) and carbon monoxide. The hydrogen contained in the reformate is used as fuel for operating the fuel cell system. Such fuel cell systems are used, for example, for powering electric motors in motor vehicles.
U.S. Pat. No. 5,344,721 discloses a gas generation system for a steam reformer with a connected fuel cell system. The system evaporates the components to be supplied to the reformer in several steps. Water is initially preheated in the cooling loop of the fuel cell, then partially evaporated through heat exchange with the hot reformate exiting the reformer, and subsequently completely evaporated through heat exchange with a burner that heats the reformer. A heat exchange with the cooling loop of the fuel cell is sufficient to evaporate the required alcohols.
Likewise, U.S. Pat. No. 4,994,331 discloses evaporation of fuel and water through heat exchange with the fuel cell coolant and the reformate flow.
U.S. Pat. No. 4,976,747 discloses the evaporation of fuel through heat exchange with reacted hot fuel cell exhaust gas.
The above methods for evaporating the components to be supplied to the reformer do not address effects caused by load changes. The service life of reformer catalysts depends, among other things, on the inlet conditions of the components (educts). Inlet conditions that are not constant and instead vary over a wide temperature range during a load change significantly reduce the service life of the reformer catalysts.
Moreover, components, such as gas cleaning (shift and selective oxidation stages) have to be designed for the worst possible operating conditions, which increases the overall dimensions and weight as well as manufacturing costs. During load changes, the vapor state (vapor temperature) can vary considerably due to the time delay between the components to be evaporated and the required and available heat content of the heating means. It has been observed that variations in the steam temperature significantly reduce the expected service life of the reformer catalysts.
It is therefore an object of the present invention to provide an improved gas generation system and a method for producing a gas flow to be supplied to a reformer, which effectively counteracts the decrease in the expected service life of the reformer catalysts caused by load variations.
This object is addressed by a gas generation system and by a method according to preferred embodiments of the present invention.
According to the present invention, a normalizing stage is connected between the evaporator and the reformer for equalizing the temperature distribution in the gas flow to be supplied to the reformer. The normalizing stage equalizes the temporal temperature valleys and peaks. According to the present invention, the temperature of the gas flow is equalized to within a temperature range that is lower than the maximal allowable reformer inlet temperature. This guarantees that large temperature variations of the evaporated components are equalized before these components enter the reformer, even in the event of a load change.
Advantageously, the normalizing stage of the present invention can be connected to conventional evaporators with several evaporator stages after the last evaporator stage and before the reformer. Optionally, additional normalizing stages can be connected between the evaporator stages.
The temperature profile of the gas flow supplied to the reformer can be equalized in different ways. For example, the normalization stage can be implemented as an adiabatic stage in which fuel and air are reacted adiabatically by a catalyst. The exothermic reaction heats the adiabatic stage. The temperature of the gas flow leaving the adiabatic stage has to be below the maximum permissible inlet temperature of the reformer. For this purpose, the air flow to the adiabatic stage is controllably metered as a function of the temperature. Additional fuel can be supplied externally to replace the spent fuel. Metering the fuel supplied to the gas flow to be reformed obviates the need for additional fuel metering in the catalytic burner of the secondary side, thereby reducing emissions.
In addition, the normalization stage can have the form of a simple heat exchanger that brings the mixture to be supplied to the reformer to a temperature below the maximum allowable inlet temperature of the reformer. Fuel cell exhaust gas or hot reformate can be supplied to a secondary side (heat source) of the heat exchanger. It will be understood that other heat sources can also be employed.
Alternatively, the normalization stage of the present invention can be in the form of a catalytically heated reactor which has (1) a primary side through which gas flows to the reformer, and (2) a secondary side that is used to heat the gas flow. A burnable gas is catalytically reacted with air in the secondary side of the catalytic reactor, wherein like the above-described adiabatic stage, the burnable gas/air can be controllably metered as a function of the temperature. For this purpose, a temperature sensor is provided in the outlet line of the primary side of the catalytic reactor. The temperature sensor is connected with a control unit that controls a valve supplying the burnable gas/air mixture. The burnable gas can be the gas to be reformed or hydrogen, and derived, for example, from the fuel cell exhaust gas.
The present invention increases the service life of the reformer catalysts. At the same time, the dynamics of the reformer process can be enhanced without lowering the life expectancy of the reformer catalysts. This allows a more dynamic operation with longer maintenance intervals.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the present invention when considered in conjunction with the accompanying drawings.